Does trainline.com offer reliable information?

https://community.ricksteves.com/travel-forum/italy/italian-trains-a862a46f-03e1-400c-8f93-aa47f3f9d1b1

Even tho this link refers to Italian trains, there is an animated discussion regarding your question.

Maybe, it depends on which train tickets you are looking at. When train tickets go on sale varies a lot depending on train company and country, Renfe especially is notorious for not releasing them well in advance. Their competitor Iryo on the other hand has their sepetember tickets on sale already, and you can buy a ticket from Barcelona to Madrid for €18 at www.iryo.eu

Without directly getting involved in the 3rd party resellers debate, why on earth do you need to book a train 6 months ahead of time, especially on a route like this, which has plenty of capacity between three different operators, and with coach competition.
It is simply not possible that, a few weeks in advance (let alone 60 days) there will not be a train at about the time you want to travel at a competitive price.
Trains are not airlines where you book half a year or a year in advance.
Does in fact trainline offer all 3 operators?
If the tickets are not on the operator's websites then they have not on issue yet. So trainline are selling you a promise that they will be able to get tickets when issued, Yes schedules are unlikely to change significantly, but they quite simply are guessing schedules until RENFE or the other operators issue timetables. So it is an educated gamble, to not get the ticket now anyway.

So now I'm wondering if that information is reliable. Can I actually book train tickets this far out and expect the information to be accurate?

We are in Spain at the moment and used the Trainline to book some of our trains. If you are looking at companies like Ouigo or Iryo then the six months out should be accurate. Renfe, the Spanish national railway, doesn't usually sell tickets more than 60 days in advance, but a site like the Trainline is probably showing you trains that are highly likely to be running based on known timetables etc.

Some of our trains didn't show up using the Trainline but could be booked directly through the Renfe website/app. These were regional trains, not the high speed ones.

Thanks everyone for all your comments. Perhaps I should clarify - I just wanted to get an idea of ticket prices in September and October so we could budget accordingly. Everything I have read or watched online has led me to believe that I would not be able to gather ticket information until about 60 days in advance. I appreciate all your information and experience-sharing. This travel forum is a valuable tool for trip planning!

I was looking on trainline.com to get an idea of train ticket prices from Barcelona to Madrid.

Iryo and Ouigo are high speed competitors to Renfe. We found Iryo to be MUCH cheaper than Renfe (about a third the price) and having now travelled on both Renfe and Iryo high speed trains there is no meaningful difference in the experience. About the only difference is that the Renfe seats are slightly more padded. Both travel on the exact same route and depart and arrive at the same stations and take pretty much the same time.

Pricing seems to vary a lot depending on the time of the day. For Renfe the advance discounts are substantial - around 25-30%.

The advantage of using sites such as Trainline, Omio and others is that they are going to show you the train schedules and prices of multiple train companies all in one listing. There now are four different train companies in Spain that run high-speed trains through the country. The Renfe website is not going to show IRYO’s or Ouigo’s train schedules or prices, for example. So, checking a reseller’s website can give you an overview of what is available schedule-wise and price-wise. Buying in advance gives you an advantage price-wise, particularly if you can commit to a non-refundable ticket months in advance. The EU has forced the state-run train companies, such as Spain’s Renfe, to allow private train companies to use their tracks and open up the train system to competition. When Renfe had a monopoly on train travel in Spain, it could charge €60-€170 for tickets on its high-speed AVE trains between Madrid and Barcelona. Today, if you buy a non-refundable ticket months in advance from one of the other train companies or resellers, you can get a high-speed train ticket between Madrid and Barcelona for as little as $8 (that’s right- eight dollars) ! This is why savvy travelers buy their train tickets way in advance, With travel insurance covering trip delays and cancellations, financial risk is minimized as policies that you would buy to have health insurance will also cover your nonrefundable travel expenses in certain situations.
Trainline and other resellers are able to provisionally sell tickets in advance of the schedules officially being released on June 10 because their computers are interlinked with the ticketing systems of companies operating the trains. “The Trainline,” RailEurope and Omio all connect with the ticketing systems of Renfe. These companies are not guessing what the train schedules and prices of tickets are for the trains—- they know what they are by connecting with Renfe’s and other train companies’ computerized ticketing systems. In Italy, the government’s national train company, called Trenitalia, allows some ticket resellers to interlink with Trenitalia’s ticketing system showing its schedules and prices. While the national train company may be prevented by government regulations from offering tickets for sale before the new train schedules are issued the second Saturday of each June and each December— the ticket resellers such as “The Trainline” and Italiarail are not restricted from doing so. And since they are privy to the information— they are free to provisionally offer the tickets for sale even before Trenitalia is able to,
The author of the website “The Man in Seat 61,” a retired BritRail employee named Mark Smith, gives a thumbs up to both The Trainline company and RailEurope as reputable and reliable ticket resellers for European train tickets.